"Life, Liberty and the pursuit of Happiness." is such a wooly useless statement though.
You could argue that based on the above, the US shouldn't have a minimum age limit for alcohol since it clearly infringes on teenagers (heck even college kids) pursuit of happiness.
I'm not arguing that it's correct that they have strict laws about pornography, nor would I argue that it's incorrect that the US has strict laws on alcohol. It's up to each country where to set its own laws.
I'm not being sarcastic. I'm a libertarian (i.e. classical liberal). I think people should have freedom to do things that don't hurt others.
It's only legitimate to stop people drinking when there's some danger involved, e.g. drinking with driving. One can argue for other cases like drinking heavily with a "mean drunk" personality type, or drinking heavily without the experience/knowledge to be safe in that situation. Drinking with being aged 20 isn't inherently dangerous. And it's a bad (both inaccurate and ageist) proxy for the other things that actually matter.
And I am not an America, so this is not a nationalism issue.